Information is transmitted in many wireless applications using Radio Frequency (RF) signals. RF signals are formed by modulating a baseband signal (information) onto a carrier signal, the carrier signal having a frequency greater than the baseband signal frequency. Upon reception, an RF signal is finally down converted to a desired baseband frequency before baseband signal processing occurs. RF signals may be directly down converted or may be processed by one or more intermediate stages before finally being converted to a desired baseband frequency.
A heterodyne-based receiver down converts a received RF signal via one or more intermediate frequency (IF) stages until a desired baseband frequency is obtained. Each IF stage comprises a mixer, a filter and/or an amplifier stage. The output of a particular IF stage has a frequency corresponding to the difference between a local oscillation signal input to the mixer and the frequency of the RF (or IF) input signal.
A homodyne-based receiver eliminates the need for IF stages by directly down converting a received RF signal to a desired baseband frequency signal. As such, homodyne-based receivers tend to use fewer components than their heterodyne counterparts, thus resulting in size and power advantages. A direct-conversion homodyne receiver conventionally includes a RF receiver front-end which comprises a Low Noise Amplifier (LNA) for amplifying a received RF signal, a mixer for directly down converting the amplified RF signal to a desired baseband frequency signal and an impedance matching circuit for matching the output impedance of the LNA to the input impedance of the mixer. In full-duplex homodyne receivers, a surface acoustic wave filter is conventionally included to filter interference injected from the transmitter to the receiver during simultaneous operation.
Various challenges exist when directly down converting an RF signal to a desired baseband frequency signal, particularly when the components used for direct conversion are integrated onto the same silicon die using a deep sub-micron CMOS technology. Deep-submicron CMOS technologies offer low supply voltages. A low supply voltage limits the linear operating range of an LNA. That is, a limited supply voltage limits the output voltage range of the LNA, thus causing it to desensitize the gain for weak wanted signals. As such, the LNA operates in a nonlinear region when processing high level signal components of an RF signal unless the gain of the LNA is adjusted downward. To avoid desensitization of the gain for weak wanted signals, good linearity is required for the RF receiver front-end. Normally, passive mixers provide better linearity than active mixers. However, passive mixers become difficult to use because passive mixers have negative gain and the total gain provided by the LNA and mixer is not enough, which results in low RF receiver front-end gain. Low RF receiver front-end gain requires that circuits following the mixer must be designed with low noise circuitry.
To compensate for reduced LNA gain, active mixers are conventionally used. Although active mixers provide additional gain, they consume more power than their passive mixer counterparts. Active mixers also have poor linearity. Further, flicker noise is present in active mixers. Although flicker noise does not affect high frequency signals, it does adversely affect low frequency signals such as narrowband GSM baseband signals. Flicker noise may be reduced by increasing the size of transistors included in an active mixer. However, larger transistors consume more power. In addition to power consumption and flicker noise concerns, active mixers also have a DC offset, thus causing level shift at the mixer output. Passive mixers cure many of the disadvantages associated with active mixers. However, passive mixers generally have negative gain. As such, it is increasingly difficult to use traditional passive mixers and LNAs using deep sub-micron CMOS technologies due to the low voltage margin associated with such technologies and insufficient gain.